Combustion of coal is commonly used for industrial heating applications. For example, coal fired electrical power plants burn coal to generate the steam used to drive their electrical generators. Coal combustion is also often used in other industries having significant heating requirements, such as metal works, cement plants, and the like.
Industrial applications using coal-fired combustion generally require the coal to be pulverized into a finely ground powder before it is ignited in the combustion chamber of a coal burner. The finely ground coal powder burns more efficiently and tends to produce fewer noxious byproducts than other forms of coal. However, powdered coal presents a significant explosion risk. In order to minimize the risk of explosion, the coal is typically ground into a powder by an on site coal mill just before the coal is fed to the burner.
One prior mill system used to grind coal into a powder and meter powdered coal to a coal burner is referred to as a negative pressure mill system. In a typical negative pressure mill system, an exhauster downstream of a mill housing draws hot air into the mill housing through an air inlet. A milling unit in the mill housing includes one or more milling elements rotated by a drive system in a manner that grinds the coal into fine particles. Coal is fed to the milling unit through a coal inlet and ground by the milling unit in the housing into powdered coal. The exhauster draws air and finely ground coal particles from the housing through an outlet of the mill housing. A classifier in the mill housing causes larger coal particles that are entrained in the air flow in the housing to fall back into the milling unit for further grinding, while at the same time allowing finely ground coal particles to remain entrained in the air flow exiting the mill housing. The exhauster has an outlet connected to the coal burner so that the mixture of air and powdered coal is blown into the combustion chamber of the burner by the exhauster. The suction from the exhauster maintains the pressure in the housing at a relatively low pressure, which is why coal mills of this type are referred to as negative pressure mills.